copyrighting material?

My method of editing as I went has paid off, I'm almost ready to send out my work. (when I complete the perfect query letter) Do I need to have my novel copyrighted before sending it to agents? I haven't found any info on this, I assumed I would, but if there's something that comes up that needs to be changed, I would think you wouldn't be able to alter it after it's been copyrighted. Or maybe I'm just not grasping this concept properly. Thanks.

Like Cogito says. I sent the Library of Congress a query about this once. Say you copyright your work now, but make changes before hitting the presses, you (or your publisher) will have to register a new copyright "for derivative woks". I was told that this derivative copyright is substantially more expensive than the initial copyright.

Thank you all. My husband has been riding me to get it copyrighted. He thinks someone is going to steal it, but I tell him that I have the massive piles of drafts and notes and journals, so I could easily prove I wrote it even in the unlikely event that it happened. Thanks again, I'm off to those sites, thanks mammamaia!

what you've done, re keeping early notes/drafts IS the best of all proof of authorship...

and before anyone brings up that old wive's tale, 'the poor man's copyright' [mailing to yourself], know that it's not admissible in court in the us, as it can be too easily faked... however, i hear it does have some standing in the uk...